It is hard-coded to the kernel, although drivers do not need to be loaded from there. Here are some bits taken from various parts of the I / O subsystem that load drivers:
#define SYSTEM32_DRIVERS_DIR (L"\\System32\\drivers\\")
...
pathLength = sizeof(L"\\SystemRoot\\System32\\Drivers\\") - sizeof(UNICODE_NULL);
path = L"\\SystemRoot\\System32\\Drivers\\";
...
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